1. Field of the Invention
The invention relates to printed conductive patterns. In particular, the invention concerns a method for forming conductive patterns on planar substrates. In the method, conductive material is applied on the surface of the substrate and the material treated so as to form a static pattern on the substrate. The invention also concerns an apparatus for carrying out the method.
2. Description of Related Art
Printing of electronics, especially on flexible substrates, pursues electronic components and applications for logistical solution, disposable electronics, even to printed displays. Nowadays, the printed electronics applications utilize methods familiar in traditional manufacture of electronic components such as electroplating and screen printing. Unfortunately, these methods are slow, not well suitable for porous substrates and/or substrates in web form. Also flexo and rotogravure printing has been used to print electronic components. Problems with these above mentioned methods are that they create structures with discontinuity (due to halftone dots), solvent evaporation and absorption of components in porous structures (with liquid materials). The main challenge with conductive polymers are insufficient electric conductivity and protection against effects of environment e.g. oxidation.
In WO 04/080138 there is disclosed one alternative method for forming electronic circuits on a substrate. In the method, metal powder is deposited as a layer on the substrate and some parts of the deposited surface are subjected to compression. At those areas, the powder is pressed into the substrate with a high-pressure hydraulic press in order to form a permanent conductor structure. The press is active for several seconds, even minutes. Thus, the method in relatively slow and involves using very high pressures, and therefore also massive equipment. The method is also based on deformation (compressibility) of the substrate. This causes mechanical weakening of the substrate, in particular, when paper substrates are concerned. Somewhat similar method, with the exception that it concerns forming uniform conductive layers, for example, for EMC shielding, is disclosed by EP 0297678.
Another methods for the manufacture of printed wiring boards by electrostatic printing are disclosed in U.S. Pat. No. 4,698,907 and WO 01/45935. WO 95/14260, on the other hand, discloses a buried electrode drum for electrostatic transfer of toner particles to surfaces.
U.S. Pat. No. 5,817,374 discloses a method for transferring charged particles on a receptor material as a thick layer by using a mask on the receptor. That is, the particles are first transferred through selective holes in the mask to the receptor and then the mask is removed from the receptor surface. The surface of the receptor may be coated or the particles may by polymerized or crosslinked on the surface in order to secure the layer to the receptor. The use of screens or masks of any kind is undesirable when the speed and resolution of particle transfer is concerned.
US 2002/0034617 discloses a method for transferring wet conductive toner to a wiring board by first adhering toner to cross-linked regions of a reusable imaging surface and then transferring the toner to the wiring board by a corona discharge. On the board the toner dries, after which it is subjected to oven treatment. In the last step, a copper layer is deposited on the dried toner in order to achieve good electrical conductivity. Thus, the method requires a large number of processing steps.
EP 0034817 concerns applying a mixture comprising metal particles and toner particles having adhesive properties when subjected to high temperature on a surface of an inert substrate. The mixture of the particles is then fired in order to finish the product. As a disadvantage in this method is that, as an auxiliary agent, adhesive toner is required in order to form the desired conductive pattern.
JP 11298119 discloses a copper-containing powder, which can be applied on ceramic substrates for forming electrical circuits. The powder includes also pressure-sensitive polymer and charge control agent. The powder is attached to the substrate by kneading. The method is not suitable for flexible substrates and does not allow using of low temperatures.
In addition, several techniques for printing conductive patterns using conductive pastes, gels and inks are known. However, application of liquid-phase materials is undesirable due to their complex processing and relatively low conductivities and/or resolution of the printed traces. Removal of the solvents or auxiliary agents initially included in the composition requires time-consuming steps.
The methods referred to above set certain limits to substrates which can be used in the methods. None of them is well suitable for paper substrates due to very high temperatures or substrate-weakening local compression used. On the other hand, deposition masks, stencils or screens are not desirable as far as speed, customizability and flexibility of the process is aimed at, since they make the deposition process unnecessarily complex and limit the obtainable resolution.